1. Field of the Invention
The invention relates to an electrostatic imaging system and, more specifically, to an apparatus and associated method for transferring a toner image from an image-forming medium to a receiving material and removing residual impurities from the imaging system so as not to contaminate the image-forming medium.
2. Discussion of Related Art
U.S. Pat. No. 4,607,947 describes a contact fixing device in which a toner image is transferred from an image-forming medium to a heatable intermediate medium. In a fixing zone, in which the intermediate medium is in contact with a biasing means, the toner image is then transferred and simultaneously fixed on a receiving material being transported through the fixing zone. However, impurities may be transferred from the receiving material to the intermediate medium. Toner material residues may also remain as an impurity on the intermediate medium because of incomplete transfer of the toner image to the receiving material. If impurities of this kind are left on the intermediate medium, they may be subsequently transferred to the image-forming medium in the first transfer zone. This causes disturbance to the image-formation and hence, ultimately, image errors in the copy on the receiving material.
Various cleaning means have been proposed to remove these impurities from the intermediate medium before reaching the first transfer zone. For example, U.S. Pat. No. 4,607,947 discloses a cleaning means having a cleaning surface to which toner adheres better than to the intermediate medium. A cleaning means of this kind operates satisfactorily for removing high-melting impurities such as toner residues. This cleaning means can also be used to remove paper dust from the intermediate medium. In practice, however, it has been found that low-melting impurities from receiving materials, such as wax-like compounds, plasticizers, anti-foaming agents, plastic fillers occurring in receiving papers, and dust particles from plastic receiving materials, and the like, are not removed or only partly removed from the intermediate medium with known cleaning means. After being deposited on the intermediate medium in the second transfer zone these impurities can also be transferred in the first transfer zone to the image-forming medium, resulting in disturbed image-formation and hence, ultimately, image defects in the copy on the receiving material. This necessitates regular and premature replacement of the intermediate medium and image-forming medium, involving high maintenance costs and equipment downtime. For example, it has been found that the increasingly used "alkaline" receiving papers based, inter alia, on cellulose, chalk and sizing agents, such as alkyl ketene dimers, are an appreciable source of such impurities. Such receiving papers are used today because of the lower costs and better durability in comparison with the "acidic" receiving papers based, inter alia, on cellulose, clay and modified or unmodified resins. It has now been found that where such receiving papers are used reaction products of these binders are deposited from these papers in the top layer of the intermediate medium. These reaction products are then transferred to the image-forming medium resulting in image disturbance.
To remove these reaction products from the top layer, U.S. Pat. No. 5,361,126, the entire content of which is herein incorporated by reference, proposes to remove the impurities via the rear of the top layer to a rubber intermediate or underlayer. An impurity-absorbing material, such as carbon black, is mixed in this intermediate layer. It is also proposed to bring transport rollers into contact with the intermediate or underlayer on the side thereof remote from the top layer, so that further transport of the impurities from the top layer can take place. U.S. application Ser. No. 093,657, filed Jul. 20, 1993, now U.S. Pat. No. 5,521,687, the entire contents of which is herein incorporated by reference, proposes using a cleaning means which between the second transfer zone and the first transfer zone can be brought into contact with the top layer of the intermediate medium, the outside of the cleaning means being provided with an impurity-absorbing material. In the devices described in the above patent documents, the amount of impurities in the top layer is reduced, but a problem still remains in that after some time the intermediate layer or underlayer becomes saturated with impurity-absorbing material and the cleaning means becomes saturated with the impurities originating from the receiving materials, thus necessitating premature replacement of intermediate medium and/or cleaning means.